Transistors formed over flat plates such as glass substrates and typically used in liquid crystal display devices generally include semiconductor materials such as amorphous silicon or polycrystalline silicon. Although transistors including amorphous silicon have low field effect mobility, they can be formed over large glass substrates. In contrast, although transistors including polycrystalline silicon have high field effect mobility, they need a crystallization process such as laser annealing and are not always suitable for large glass substrates.
On the other hand, transistors including oxide semiconductors as semiconductor materials have attracted attention. For example, Patent Documents 1 and 2 disclose techniques by which a transistor is formed using zinc oxide or an In—Ga—Zn—O-based oxide semiconductor as a semiconductor material and is used as a switching element of an image display device.
Transistors including oxide semiconductors in channel regions have higher field effect mobility than transistors including amorphous silicon. Further, oxide semiconductor films can be formed at a temperature of 300° C. or lower by a sputtering method or the like; thus, a manufacturing process of transistors including an oxide semiconductor is simpler than that of transistors including polycrystalline silicon.
Such transistors including oxide semiconductors are expected to be used as switching elements included in pixel portions and driver circuits of display devices such as liquid crystal displays, electroluminescent displays, and electronic paper. For example, Non-Patent Document 1 discloses a technique by which a pixel portion and a driver circuit of a display device include transistors including oxide semiconductors.
Note that transistors including oxide semiconductors are all n-channel transistors. Therefore, in the case where a driver circuit includes transistors including oxide semiconductors, the driver circuit includes only n-channel transistors.    [Patent Document]    [Patent Document 1] Japanese Published Patent Application No. 2007-123861    [Patent Document 2] Japanese Published Patent Application No. 2007-096055    [Non-Patent Document]    [Non-Patent Document 1] T. Osada et al., “Development of Driver-Integrated Panel using Amorphous In—Ga—Zn-Oxide TFT”, Proc. SID '09 Digest, 2009, pp. 184-187.